GLUT4 trafficking in insulin-sensitive cells

�9 1999 by Humana Press Inc. All rights of any nature whatsoever reserved. 1085-9195/99/30/89-113/$16.25

GLUT4 Trafficking in Insulin-Sensitive Cells

A Morphological Review

Sally Martin/Jan W. Slot, 2 and David E.James *,1

1C entre for Molecular and Cellular Biology, and Department of Physiology and Pharmacology, University of Queensland, Brisbane, OLD 4072,

Australia, E-mail." [email protected]; and eDepartment of Cell Biology, Utrecht University Medical School,

Utrecht, The Netherlands

ABSTRACT

In recent years, there have been major advances in the understanding of both the cell biology of vesicle trafficking between intracellular compartments and the molecular targeting signals intrinsic to the trafficking proteins themselves. One system to which these advances have been profitably applied is the regulation of the trafficking of a glucose transporter, GLUT4, from intracellular compartment(s) to the cell surface in response to insulin. The unique nature of the trafficking of GLUT4 and its expression in highly differentiated cells makes this a question of considerable interest to cell biologists. Unraveling the tangled web of molecular events coordinating GLUT4 trafficking will eventually lead to a greater understanding of mammalian glucose metabolism, as well as fundamental cell biological prin- ciples related to organelle biogenesis and protein trafficking.

Index Entries: Intracellular trafficking; insulin action; glucose transport; GLUT4; muscle; adipose tissue.

*Author to whom all correspondence and reprint requests should be addressed.

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90 Martin, Slot, and James

ABBREVIATIONS

ANF, atrial natriuretic factor; SR, sarcoplasmic reticulum; TGN, trans-Golgi network/ t rans-Golgi reticulum; TfR, transferrin receptor; T-tubules, t ransverse tubules; TV, tubulo- vesicular.

INTRODUCTION

Glucose is a major source of metabolic energy for most animal cells. However, because it is a polar molecule, it is unable to pen- etrate the plasma membrane without the aid of a glucose transport protein. Mammalian cells express distinct glucose transporters dependent upon their individual requirements for glucose and consistent with their physiological function (1). All glucose transporters have similar secondary structure and membrane topology, consist- ing of twelve membrane-spanning o~-helical domains, intracellular N- and C-termini, a large central cytoplasmic loop between helices 6 and 7, and a single N-glycosylation site within the largest extracellular loop between helices I and 2 (1,2). The highest degree of sequence identity between the transporters is found within the transmem- brane domains, implicating a functional role for these regions in the formation of a glucose pore or binding site (3). In contrast, the unique amino-acid sequences within the extramembranous domains suggests that these regions play a role in the differential regulation and kinetics of the transporters.

As early as 1939, it was proposed that insulin increases the transfer of glucose from the blood to the interior of the cell (4). In 1980 it was demonstrated by cytochalasin B binding and subcellular fractionation, that the mechanism by which this occurs is via the insulin-dependent translocation of an intracellular population of glucose transporters to the cell surface following insulin stimulation (5,6). The glucose transporter isoform, GLUT4, which realizes this effect has been extensively studied owing to its potential role in insulin- resistant disease states, such as Type II diabetes mellitus. Tissues which exhibit insulin-regulated glucose transport, namely skeletal muscle, cardiac muscle, and fat, express the highest levels of the GLUT4 protein (7). In non-stimulated cells GLUT4 is almost completely localized intracellularly (7,8). In response to a stimulus such as insulin it translocates to the cell surface. When the stimulus is

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GLUT4 Trafficking 91

removed GLUT4 is re-internalized and returns to an intracellular storage site. The increased glucose transport capacity of the cell following translocation of GLUT4 to the cell surface is the basis for the increased transfer of glucose from the blood to the interior of the cell in response to insulin.

The intracellular localization of GLUT4 in non-stimulated adipocytes and muscle cells is fundamental for the acute response to insulin. In contrast, both GLUT1 and GLUT3, which are also intracellular in some tissues, are additionally present on the cell surface. GLUT3 is expressed in numerous cell types, including neurons, sper- matozoa, platelets, embryos, and tumor cells of epithelial origin (7,9- 11). In platelets GLUT3 is predominantly present in the m-granules of the intracellular regulated secretory compartment, from which it translocates to the plasma membrane in response to a stimulus such as thrombin (11,12).

Although GLUT4 recycles constitutively (12), kinetic studies have indicated that the internal localization of GLUT4 is largely facilitated via retention of the protein at an intracellular location. The reported rate of GLUT4 exocytosis (kex) in basal adipocytes varies between 0.01/min and 0.024/min (12-14) and is considerably lower than that of other constitutively recycling proteins such as GLUT1 (kex = 0.035/min) (12) and the transferrin receptor (kex = 0.111/min) (15). Following insulin stimulation the rate of exocytosis of GLUT4 is increased to between 0.32/min and 0.86/min (13-15), and the rate of endocytosis is reported by some researchers to decrease (13) although this latter effect appears quantifiably less relevant. The importance of retaining GLUT4 in an intracellular compartment has prompted an investigation into its nature. This includes defining co-inhabitants of the compartment and dissecting at a molecular level how these proteins are addressed to this par- ticular intracellular location.

INTRACELLULAR TRAFFICKING PATHWAYS

One of the best-characterized systems of protein trafficking between the cell surface and the cell interior is that of the transferrin receptor (TfR) (Fig. 1A). TfR binds to iron-loaded transferrin at the plasma membrane and is internalized by clathrin-mediated endocytosis. Following dissociation of the clathrin-coat, these vesicles fuse with early/sorting endosomes, where the low pH induces dissocia-

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tion of iron from transferrin. The receptor, usually with transferrin still bound, recycles back to the cell surface. Most TfR exocytosis occurs via transport vesicles that bud directly from the sorting endosome (fast cycle). However, a proportion traffics back to the cell surface through recycling endosomes (16,17). Recycling endosomes are distributed throughout the cell although frequently concentrated in a pericentriolar location (18) and have a tubulo-vesicular profile in cell sections. Additionally a small amount of the TfR traffics back to the trans-Golgi-network (TGN) (19), a tubular/reticular organelle immediately adjacent to the trans-Golgi cisternae (20,21). The TGN is the site of sorting of newly synthesized membrane proteins, which recycle between intracellular compartments such as the mannose 6-phosphate receptors, lysosomal proteins, and of secretory proteins. The TGN can be morphologically defined on the basis of its proximity to the trans-Golgi cisternae, but its appearance often makes it hard to distinguish from endosomal elements. This distinc- tion is often made with the use of the marker proteins TGN38 (22,23) and furin (24-26). The precise relationship between the TGN and recycling endosomes is not known.

Despite the relative simplicity of the TfR recycling system, evidence points to greater complexity with the endosomal systems themselves. The perception of the endosomal system as discrete, relatively simple organelles has evolved to include different types of sorting endosomes (27, 28), the presence of separate recycling systems (16,17,29,30), and even possible heterogeneity within the membranes of the TGN (31). It is known that in specific cell types specialized endosomes can be generated to perform specific tasks. Examples of this include the class II compartments of B-cells (32,33), where endocytosed exogenous proteins are proteolyically degraded generating peptides which are then bound to trafficking MHC class II proteins before the peptide-MHC complex is presented at the cell surface. Alternatively, vesicles derived from macropinocytosis, which frequently occurs at the leading edge of cells, appear to deliver ligands to a different popula t ion of endosomes than c la thr in-mediated endocytosis (34). Most intracellular proteins that are known to traffic through the cell have been shown to contain multiple, intrinsic targeting motifs (35). Regulated recycling proteins such as GLUT4 could therefore be retained in a specialized endosomal compartment in the absence of insulin.



THE INTRACELLULAR GLUT4 COMPARTMENT

Proteins found in GLUT4-containing vesicles include the mannose 6-phosphate receptors (36,37), the TfR (36,38), transferrin (38), synaptobrevins (39-41), SCAMPs (42) and GLUT1 (43,44). However, in general insulin only generates a two- to threefold increase in the cell surface levels of these proteins, in contrast to the >10-fold increase in GLUT4. One reason for the wide range of proteins identified in GLUT4 vesicles, and the considerable disparity between the results, could be the inability of immunoprecipitation techniques to distinguish between vesicles enriched in GLUT4, and compartments with which GLUT4 is only transiently associated. Translocation of proteins such as GLUT1 and the TfR can also be triggered by a range of other stimuli including stress (45) and growth factors (46), in a range of cell types including fibroblasts. This suggests that regulated recycling through the endosomal system may be a general charac- teristic of all cells. To date, the only protein to show an increase at the cell surface equivalent to that of GLUT4 in response to insulin is an aminopeptidase, vp165, one of the major protein constituents of GLUT4 vesicles (47-53). A number of studies, described below, have investigated the morphological distribution of GLUT4 in insulin- regulated cells.

LOCALIZATION OF GLUT4

Muscle and adipose tissue express very high levels of the GLUT4 protein. Structurally however, these two tissues are very different, each being highly specialized for its respective function. The rat adipocyte is up to 120 ~tm in diameter. However, the cytoplasm occupies only about 1/40th of the cell volume. The remainder of the cell is composed of a single, large lipid droplet. Myocytes are also extremely large cells. However, again a large proportion of the muscle cell volume is filled with bundles of myofibers. The classic, intracellular trafficking pathways are difficult to conceptualize in a cell in which the recycling GLUT4 protein could be many microns away from the classical recycling compartments, such as the peri- nuclear TGN and/or the pericentriolar recycling endosomes. Recently, studies in adipocytes have shown that markers of the TGN and Golgi can be distributed widely around the periphery of these cells (55), suggesting that classic trafficking pathways could exist in regions of the cell distal to the nuclear region.

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A reasonably complete picture of GLUT4 distribution has come from looking at brown adipose tissue (54,55). Brown adipose tissue offers several advantages compared to other insulin-sensitive cell types:

1. The multilocular lipid droplets can be reasonably depleted, which is advantageous for morphological studies (54);

2. These cells express very high levels of GLUT4 (56), thus affording greater sensitivity for immunolabeling; and

3. Brown adipose tissue is exquisitely insulin sensitive (57,58) thus enabling a detailed quantitative study of changes in GLUT4 distribution in response to stimulation.

Localization of GLUT4 in Brown Adipose Tissue

In brown adipose tissue from fasted rats GLUT4 labeling was found in the TGN, and in small, predominantly noncoated, tubulo- vesicular (TV) profiles close to the plasma membrane or distributed throughout the cells (54). There was little labeling detected on the plasma membrane, in early/sorting endosomes (defined by the presence of endocytosed rat albumin), late endosomes (defined by the presence of cathepsin D), Golgi cisternae, or endoplasmic reticulum (54,55). However, 1.1% of the GLUT4 labeling was present in clathrin-coated vesicles. This is equivalent to the amount of GLUT4 labeling on the entire plasma membrane in the basal cell. Hence, in agreement with most (59,60), but not all (66), studies in other insulin-sensitive cells, GLUT4 is relatively enriched in coated pits and vesicles. In response to insulin a reduction in GLUT4 labeling of around 40% was observed in both the TV elements and the TGN, and there was a concomitant increase in labeling of the plasma membrane. Increased labeling was also observed in clathrin-coated pits and vesicles, and in sorting endosomes.

It has been suggested in white adipose tissue (61-64) that GLUT4 is concentrated in caveolae: small, detergent-resistant invagi- nations of the plasma membrane containing a protein, caveolin (65). However, other studies could not confirm this (66). In brown adipose tissue, GLUT4 is not concentrated in caveolae (54) and GLUT4 does not colocalize with caveolin in adipocytes (67). Furthermore, detergent insoluble complexes devoid of known caveolins have been isolated from several cells suggesting that detergent insolubility is itself an insufficient marker of these domains (68,69). Most of the evidence to date points to clathrin-mediated endocytosis as the


GLUT4 Trafficking

A B

95

Fig. 1. Protein trafficking pathways: constitutive trafficking of the TfR and regulated trafficking of GLUT4. (A) Recycling of the TfR. The TfR is internalised through clathrin-coated pits [1] and traffics to the early/sorting endosomes [2]. From the early endosomes most TfR returns directly to the cell surface (indicated by the thick arrow), whereas a small proportion returns to the cell surface via the recycling endosomes [5, indicated by the thin arrow], and a proportion recycles through the TGN [6]. (B) Regula- ted recycling of GLUT4. GLUT4 is internalised through clathrin-coated pits [1] and enters the early/sorting endosomes [2]. From the sorting endosomes GLUT4 traffics to TV-elements distributed throughout the cell [5] or in the region of the TGN [6], either directly or, observed in ventricular cardiomyocytes, via multivesicular endosomes [3] (60). In the basal state, -13% of the GLUT4 is in the TGN, -63% is in the TV-elements and the remainder is elsewhere in the cell. Following stimulation with insulin the amount of GLUT4 label in the TGN and the TV-elements is decreased by -50%, with a concomitant increase in the labeling of the plasma membrane (up to ~35%) and of both early endosomes and clathrin-coated pits and vesicles (54).

major route through which GLUT4 is internalized in both the absence of insulin and following insulin stimulation.

On the basis of these observations a model of GLUT4 trafficking in basal and insulin-stimulated cells was proposed (Fig. 1B). Evi- dence points to the internalization of GLUT4 from the plasma membrane through a clathrin-mediated process(59,70). Furthermore, targeting motifs have been identified in the cytoplasmic domains of GLUT4 that resemble generic targeting motifs directing endocytosis



of proteins by clathrin coated pits (71-74). A phenylalanine-based motif in the N-terminus of GLUT4 (sequence FSQQI 8) is very similar to the tyrosine-based motifs in endocytosed plasma membrane receptors such as the mannose 6-phosphate receptor and the transferrin receptor (75). In addition, a dileucine-based motif in the C- terminus of GLUT4 (sequence L489L 490) is also similar to endocytosis motifs in receptors such as the interleukin-6 receptor (35,76). After shedding the clathrin-coat, the endocytic GLUT4 vesicles fuse with sorting endosomes. From the sorting endosomes GLUT4 enters the TV-elements, from where it traffics back to the cell surface, either directly or via intermediate endosomal compartments. The nature of the differentially distributed TV-elements and their relationship to each other is not known. Evidence from biochemical and kinetic studies suggests that there are multiple intracellular GLUT4 compartments (14,77), however, as insulin results in decreased GLUT4 labeling in TV-elements regardless of their location in the cell, clearly these structures are in dynamic contact with each other.

The nature of the TV-elements in the region of the Golgi complex is also not completely understood. Although morphologically these structures resemble the TGN (Fig. 2), GLUT4 does not colocalize with TGN38 (31,55,78) a classical marker protein of the TGN. However, GLUT4 is a prominent protein in AP-1 linked clathrin- coated vesicles derived from the TGN (S. Martin, D. E. ]ames, and G. W. Gould, unpublished observations) and is present in secretory granules formed in the TGN (79). Furthermore, as GLUT4 labeling is not decreased following treatment with inhibitors of protein synthesis (79,80) clearly the GLUT4 in this location is not derived from de novo synthesis. It is therefore likely that there are discrete regions within the TGN, presumably to accommodate the formation of distinct classes of transport vesicle containing different sets of cargo.

Other Insulin-Regulated Tissues

The GLUT4-containing structures identified in brown fat were subsequently identified in other insulin regulated tissues, including atrial (79) and ventricular cardiomyocytes (60), and skeletal muscle (81,82). These cells contain common features related to their function, the most obvious of which are the large bundles of contractile myofibers within the cytoplasm. These make up most of the volume of the cell, and in the case of skeletal myocytes displace the nuclei to the cell periphery. However, there are two other distinguishing lea-


GL UT4 Trafficking 9 7

Fig. 2. Immuno-gold labeling of GLUT4 in 3T3-L1 adipocytes. In unstimulated cells, GLUT4 labeling is found in tubulo-vesicular elements throughout the cytoplasm, including structures beneath the plasma membrane. In addition, strong GLUT4 labeling is present in TV-elements proximal to the Golgi stack, conventionally regarded as TGN. Arrow heads = plasma membrane; g, Golgi; tgn, trans-Golgi network. Size bar = 100 nm.

tures of myocytes: discrete subdomains of the plasma membrane, and a Ca 2+ storage compartment known as the sarcoplasmic reticulum (SR) (Fig. 3). Furthermore atrial cardiomyocytes also function as endocrine cells, synthesizing and secreting a peptide hormone, ANF, in response to various stimuli. Unlike adipocytes, where the plasma membrane can be effectively and efficiently purified from other cell membranes (83), subcellular fractionation of muscle is difficult, possibly due to the complex arrangement of its surface membranes or to its complex cytoskeletal system, or because of the difficulty in separating myocytes from nonparenchymal cells as well as extracellular matrix proteins prior to analysis. For this reason the use of immunocytochemistry has proved valuable.

SKELETAL MUSCLE Skeletal myocytes are large, cylindrical, multinucleated cells

that can be up to 30 cm in length and 0.5 m m in diameter. In



A

B

Fig. 3. Striated muscle: structure and plasma membrane arrangements. (A) Sarcomeric arrangement of striated muscle fibres. The myocyte structure has a periodicity related to the sarcomeric units. The T-tubule system passes through the cells at the level of the Z-line in cardiac muscle and at the interface of the A-I bands in skeletal muscle. Also in skeletal muscle (demonstrated here) the nuclei are restricted to the periphery of the cells. (B) Schematic arrangement of the T-tubule system and the sarcoplasmic reticulum in skeletal muscle. The T-tubules pass within the cells, surrounding the bundles of myofibers shown in A. Running parallel to the myofibers is the sarcoplasmic reticulum. The terminal cisternae of the SR junctions are contiguous with the T-tubules, forming a triad.


GL UT4 Trafficking 99

myocytes translocation of GLUT4 to the cell surface can be independently effected by insulin and by exercise, and the combined effects are fully or partially additive (84-87). Studies have shown that the pool of intracellular GLUT4 depleted following insulin-stimulated translocation to the cell surface is not depleted following contraction (86,88), while resolution of intracellular membranes from skeletal muscle on discontinuous sucrose gradients has identified two separate fractions which are independently sensitive to insulin or to contraction (89). Furthermore, wortmannin, an inhibitor of the signal l ing protein phosphatidylinositol 3-kinase, implicated in the insulin-stimulated translocation of GLUT4 (90-92), inhibits insulin- stimulated but not contraction-stimulated glucose transport in skeletal muscle (93). These studies suggest that in skeletal muscle the translocation of GLUT4 in response to insulin occurs through a different mechanism to that of exercise (93) and may involve distinct pools of GLUT4 (86,88,89).

The transverse (T)-tubule system is a tubular extension of the muscle plasma membrane (the sarcolemma) which penetrates deep into the interior of the cells, encircling the bundles of myofibers. The T-tubules have a very regular arrangement within the cell with two branches per sarcomere, at the A-I myofibrillar junction. The surface area of the T-tubule system has been shown to be between 0.2-0.3 ~tm 2 per unit ~tm 3 of myofibre (94) and has been estimated to be up to 9x the surface area of the sarcolemma (82). Therefore, this repre- sents a very important plasma membrane domain in these cells.

In basal muscle cells, despite quantitative inconsistencies, quali- tatively GLUT4 was identified in TGN (81,95-97), and in TV-elements which were either beneath the sarcolemma (81,82,95,97) or distributed throughout the cytoplasm, usually in the region of the triad (81,82,95-98.). The tubulo-vesicular nature of these structures closely resembled the structures shown to contain GLUT4 in brown adipose tissue. There was very little GLUT4 on the plasma membrane, or in early endosomes (81).

Stimulation of GLUT4 translocation by either insulin alone (82,95-99), exercise alone (100), or insulin combined with exercise (81) demonstra ted increased labeling of either the sarcolemma (81,95) or the T-tubules (82,96-100). As with the distribution of GLUT4 in the basal skeletal muscle, the inconsistency in the identification of the plasma membrane domain to which GLUT4 was predominantly targeted appeared to be more quantitative than qualitative.



As a general consensus it is likely that GLUT4 translocates to both the sarcolemma and the T-tubule membrane in skeletal muscle. However, the precise degree of translocation to each plasma membrane domain, and whether it is effected equally by insulin and by exercise is still open to question.

The sarcoplasmic reticulum (SR) runs parallel to the myofibers, and forms close contacts with the T-tubule system. The junction between the T-tubules and the SR forms a clearly defined triad in skeletal muscle. An additional observation in skeletal muscle was the apparent targeting of GLUT4 to the terminal cisternae of the SR (82,95). However, as these and other researchers (96) have pointed out, it is very difficult to distinguish individual membrane compartments in this region of the cell. Conflicting studies have shown GLUT4 labeling in TV-elements very close to the SR, but not functionally part of it (60,96,98). Furthermore, SR markers are detectable, but not concentrated in GLUT4 vesicles isolated from skeletal muscle (81,88).

VENTRICULAR CARDIAC MUSCLE

In contrast to skeletal muscle, cardiomyocytes are much smaller, mononucleated ceils, joined longitudinally by another specialized region of plasma membrane enriched in junctional complexes (101) called the intercalated disc. In cardiomyocytes, a single branch of the T-tubule system is positioned at the level of the Z-line, and this forms irregular contacts with elements of the SR.

In basal ventricular cardiomyocytes the localization of GLUT4 was very similar to that in skeletal muscle: concentrations in the TGN and in TV-elements both beneath the sarcolemma and close to the T-tubules (60). A combined stimulation with insulin and exercise resulted in an approximately equal increase in GLUT4 labeling on all three plasma membrane domains: the sarcolemma, the T-tubules and the intercalated discs. GLUT4 was also observed trafficking through clathrin-coated pits and sorting endosomes, consistent with it following the same intracellular trafficking pathways as previously determined in brown adipocytes (54). Furthermore, a segregation of GLUT4 from endocytosed albumin was observed in multivesicular bodies, consistent with a proposed role for these organelles in sorting endocytosed proteins destined for recycling away from proteins destined for lysosomal delivery and degrada- tion (102,103). Cell Biochemistry and Biophysics Volume 30, 1999

GLUT4 Trafficking 1 O1

ATpa~ CARDIOMYOCYTES

Several studies have considered the effect of a regulated secretory system on the targeting of the GLUT4 protein. The translocation of GLUT4 to the cell surface in response to a stimulus is in many ways similar to the regulated secretion of proteins, suggesting that the biogenesis of the insulin-responsive GLUT4 compartment is analogous to the biogenesis of secretory granules. The high concentration of GLUT4 labeling in the TGN, which is not effected by inhibitors of the biosynthetic pathway (79,80), supports this theory. Studies of GLUT4 targeting in heterologous secretory cell lines such as PC12 cells (104,105) and insulinomas (106) transfected with GLUT4 suggested that little or no GLUT4 was targeted to the secretory granules. How- ever, one study did find that most of the secretory granules (>85%) contained some GLUT4 labeling (105). An ideal model system in which to study the relationship between GLUT4 distribution and the regulated secretory system is the endocrine atrial cardiomyocyte.

Atrial cardiomyocytes are insulin regulated and express endogenous GLUT4, although in contrast in skeletal muscle they do not appear to be essential for the maintenance of glucose homeostasis. However, they also contain a regulated secretory system for ANF, a potent hypotensive hormone, secreted in response to various stimuli (107,108) including stretch, hypoxia, and thrombin (109), although elevated serum ANF levels have not been detected in response to insulin (80). In atrial cardiomyocytes 40% of the GLUT4 labeling was present in the TGN and in TV-elements throughout the cell, resembling the distribution in ventricular cardiomyocytes (60). However, more than half of the GLUT4 labeling was present on the ANF granule membranes (79). Almost all of the granules, including those in the periphery of the cell, were labeled for GLUT4, demon- strating that this was not a transient association of GLUT4 with the newly formed granules. ANF granules are unusual in that they can be seen budding from all the cisternae of the Golgi, not just in the TGN. GLUT4 labeling however, was only associated with granules budding from the TGN or from the trans-cisternae of the Golgi apparatus, and was not present in granules budding from the cis-, or medial-Golgi cisternae. The function of the targeting of GLUT4 to the ANF granules is not known. Interestingly, vp165, which is highly colocalized with GLUT4 in other insulin- responsive cell types (47-52), and in the TV-elements and the TGN of atrial cardiomyocytes, is not enriched in the ANF granules (52). Cell Biochemistry and Biophysics Volume 30, 1999


Localization of GLUT4 in Unconventional Cell Types

In addition to muscle and fat cells, GLUT4 is also expressed at low levels in various cell types in which glucose transport is not conventionally thought to be regulated by insulin. These include specific regions of brain (110), the kidney (111-114) and the stomach (J. Slot, unpublished observations). In each case GLUT4 labeling was predominantly intracellular. In the stomach GLUT4 labeled the TGN and TV-elements present in the parietal cells of the gastric glands (J. Slot, unpublished observations). In the kidney, one group of GLUT4-containing cells are the myoepithelioid cells of the juxtaglomerula apparatus (114). These cells are present in the affer- ent arteriole proximal to the glomerulus, and contain numerous secretory granules containing renin (115,116). As in the insulin- responsive cells, juxtaglomerular GLUT4 is present in intracellular TV-elements resembling the TV-elements in adipocytes and myocytes, but in contrast to the ANF granules of atrial cardiomyocytes, GLUT4 is not present in the renin granule membrane.

In all of these other cell types the labeling for GLUT4 was very low and it was not possible to demonstrate translocation to the cell surface. The functional role of GLUT4 in these other cell types remains elusive.

SUMMARY

The exclusion of GLUT4 from the cell surface of basal cells appears to be intrinsic to the GLUT4 protein itself. When GLUT4 is expressed in heterologous cell types in which glucose transport is not acutely regulated by insulin, such as fibroblasts (80,117-120) or polarized epithelial cells (121), it is present in an intracellular compartment that is morphologically similar to the tubulo-vesicular compartment in insulin-sensitive cells. However, as fibroblasts and epithelial cells are not highly insulin-responsive it is not possible to definitively conclude that an analogous compartment pre-exists in all cell types, or can be formed through the expression of GLUT4. In contrast, other glucose transporters appear to be targeted to the plasma membrane under similar circumstances (121). Studies of GLUT4 trafficking kinetics (77,122), the protein composition of GLUT4 vesicles (41,123,124), and the targeting and insulin-regulation of GLUT4 mutants (125), have shown that GLUT4 is likely to be distributed between multiple intracellular compartments in insulin- Cell Biochemistry and Biophysics Volume 30, 1999


responsive cells. However, little is known of the nature of these intracellular GLUT4 compartments and how they relate to the TV- elements containing GLUT4 identified in the morphological studies of muscle and fat cells. To date, very few intracellular proteins have been successfully localized in insulin-regulated cells. A combination of biochemical and immunologica l techniques will need to be employed to precisely define the nature of the insulin-responsive GLUT4 compartments.

ACKNOWLEDGMENTS

We wish to thank Rob Parton for his critical reading of the manuscript and helpful suggestions. DE James is a Wellcome Trust Professorial Research Fellow, and this work was supported by grants from the National Health and Medical Research Council of Australia and the Australian Diabetes Society (to DEJ).

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GLUT4 trafficking in insulin-sensitive cells